First off you have coalcanths effect. When you summon him; you can then call priority for his ignition effect to use it as chain link one if you desire.

*If you don't understand priority; then I would advice you look it up.

When his effect is activated; you discard one card for the cost (card cost's are NOT optional and must be met; whether the card is negated or not). then your opponent can chain a card that negates monster effects..so anything like Doomcaliber knight..divine wrath...royal oppression,etc

If coalcanths effect goes off without a chain; then you MUST summon level 4 or lower fish type monsters from your deck until all of your open monster card zones are filled up.

At this point..monsters are considered special summoned and your opponent has the option to activate a card in response to that.

In this case; mirror of oath would be activated to destroy all monsters special summoned by coalcanths effect, and given they came from the deck..your opponent is allowed to draw one card as per his card's effect.

At this point Coalcanth's effect has done resolved and his "once per turn" limitation has been met; so you now have to wait till next turn to re-use his effect.